[libc++] Re-apply the workaround for timespec_get not always being available in Apple SDKs

This commit re-applies 99f3b231, which was reverted in 81424257
because it broke the modules build. The modules failure was a circular
dependency between the Darwin module and __config. Specifically, the
issue was that if <__config> includes a system header, the std_config
module depends on the Darwin module. However, the Darwin module already
depends on the std_config header because some of its headers include
libc++ headers like <ctype.h> (they mean to include the C <ctype.h>,
but libc++ headers are first in the header search path).

This is fixed by moving the workaround to <ctime> only.

https://llvm.org/PR47208
rdar://68157284